The present invention relates to magnetic brush apparatus in which a development electrode is biased with composite D.C. and A.C. signals to minimize background development and flaming.
Among triboelectric developing apparatus, the most commonly used are cascade apparatus and magnetic brush apparatus. In cascade apparatus, gravity is used to roll developer across the image. Cascade apparatus generally are used in low-speed copiers. A reason for this is that in a high-speed copier, a cascade apparatus would require substantial more space than magnetic brush apparatus to effectively tone electrostatic images.
In magnetic brush apparatus which use a developer having carrier and toner particles, the carrier particles are ferromagnetic. These ferromagnetic carrier particles are held to an applicator surface, for example, a nonmagnetic cylinder, in a bristle formation, by magnets located inside the cylinder. The bristles are brushed across a surface carrying an electrostatic image. Areas of the image exerting less attractive force on the toner than is exerted by the carrier are cleaned of toner as they are brushed. Magnetic brush apparatus also may use what is known in the art as a single component developer. Such a developer is disclosed in U.S. Pat. No. 3,816,840 issued June 11, 1974 to Kotz. In U.K. patent application No. 2,073,057, a magnetic brush apparatus for a single component developer is shown. The brush is biased with an A.C. signal which according to this patent improves the tonal rendition of a developed image.
To prevent attraction of toner to background portions of the electrostatic image, it is a common practice to electrically bias an electrode provided in the magnetic brush apparatus at a voltage level approximately equal to and at the same polarity as the voltage of the background portion of the images on the image-carrying member. The resultant electric field, which acts on toner particles and effects toning of an image, is related to the difference between the charge or voltage on the photoconductor and the bias voltage placed on the electrode.
Magnetic brush development often causes flaming. Flaming is the incomplete development of the leading edge of a large solid area. Flaming is related to the velocity of moving image-carrying member, the charge V.sub.o on the photoconductor and the bias voltage. As the velocity of the member increases, flaming increases. A discussion of flaming is set forth in commonly assigned U.S. Pat. No. 4,292,921 to Kroll et al. As is disclosed in the Kroll et al patent and also in Research Disclosure No. 16126 (September 1977), a counter-current roller preceeded by a co-current roller can reduce flaming. Research Disclosure is published by Kenneth Mason Publications, Ltd., the Old harbourmaster's, Eight North Street, Emsworth, Hampshire P010 7DD, ENGLAND. A difficulty with this approach is that even though may be reduced at low speeds, when development of an image on a high-speed member is needed, there may not be sufficient toner applied to an electrostatic image to fully develop large solid areas.
In order to enhance development of electrostatic latent images and increase development of large solid images moving at high speed, many magnetic brush development apparatus include four or more brushes. See, for example, U.S. Pat. No. 3,911,864 where the magnetic brushes rotate in alternate directions, and U.S. Pat. Nos. 4,027,621, and 4,086,006, where all magnetic brushes rotate co-current. Such magnetic brush apparatus are complex, require substantial space and are expensive to manufacture.